1. Field of the Invention
The present invention relates to a compact inner focusing lens having high imaging performance.
2. Description of the Related Art
Conventionally, a relatively long flange focal length has to be established with respect to the focal length and thus, many lenses for single-lens reflex cameras adopt a configuration that includes a positive lens group toward the rear of the optical system to easily establish back focus. Nonetheless, in recent years, camera bodies have decreased in size and consequent to the spread of digital cameras, instances where long flange focal lengths are not necessary are increasing.
Further, since video can also be captured by a digital camera, high-speed autofocus processing is desirable. In autofocus processing, a portion of a lens group (focusing group) is moved rapidly along the optical axis (wobble) to achieve transitions from a non-focused state→a focused state→a non-focused state. Further, a signal component of a specific frequency band of a partial image area is detected from the output signal of the image sensor; an optimal position of the focusing group achieving a focused state is determined; and the focusing group is moved to the optimal position. In particular, when video is captured, this series of operations has to be rapidly continued, repeatedly. Further, in the execution of wobble, the focusing group has to be light-weight and have the smallest diameter possible to enable rapid driving of the focusing group.
When a positive lens group is disposed farthest on the image side of an optical system, the refractive power of the focusing group has to be somewhat strong and therefore, the optical system becomes susceptible to aberration variations consequent to wobble and the effects of magnification. To suppress aberration variations consequent to wobble and the effects of magnification when focusing is performed, disposal of a negative element farthest on the image side of the optical system is desirable.
Thus, to address such demands, an inner focusing lens that can also sufficiently cope with video filming has been proposed (for example, refer to Japanese Patent Application Laid-Open Publication No. 2013-97212).
The inner focusing lens disclosed in Japanese Patent Application Laid-Open Publication No. 2013-97212 has sequentially from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a negative refractive power, where the second lens group is moved to perform focusing. The inner focusing lens has a medium telephoto focal length by a 35 mm film camera conversion and a small, light-weight focusing group internally.
On the other hand, conventionally, at an image sensor that optically receives and converts an optical image into an electronic image signal, there are limitations on the efficiency of taking in incident light by the on-chip micro lens, etc. and on the lens side, it is desirable for the exit pupil to be made to be greater than a certain diameter and to assure telecentricity of the luminous flux incident to the image sensor.
Nonetheless, with recent image sensors, improved aperture ratios and advances in the design freedom of on-chip micro lenses have reduced exit pupil limitations demanded on the imaging lens side. Furthermore, with recent software and camera system advances and improvements, even when distortion is significant to an extent that conventionally, the distortion would be conspicuous, correction by image processing has become possible.
Therefore, in conventional image lenses, although a positive lens element is disposed farthest on the image side of the optical system and telecentricity is assured, in recent years, this is no longer necessary and even when a negative lens element is disposed farthest on the image side of the optical system and there is oblique incidence of the luminous flux on the image sensor, vignetting (shading) consequent to mismatching of the on-chip micro lens and pupil, etc. has become inconspicuous. Further, since a negative lens element can now be disposed farthest on the image side of an optical system, reductions in the diameter of optical systems can be expected.
In contrast, with the inner focusing lens disclosed in Japanese Patent Application Laid-Open Publication No. 2013-97212, since a positive lens element is disposed farthest on the image side of the optical system, which has a shorter overall length, the diameter of the third lens group (lens farthest on the image side) cannot be sufficiently reduced. Therefore, application to cameras having a smaller dimension along the direction of the diameter of the optical system is difficult, such as mirrorless interchangeable-lens cameras that have come into wide use.
The inner focusing lens disclosed in Japanese Patent Application Laid-Open Publication No. 2013-97212 is not aimed for wide angle views and therefore, the correction of field curvature and distortion as well as assuring the amount of light at the lens ends necessary for wide angle views are points that have not been considered.